FORE-GUT, MID-GUT, AND HIND-GUT. 61 



true skin which lies beneath it. The wall of the stomach 

 is a soft pale membrane containing variously disposed 

 muscular fibres ; and, beyond the pylorus, it is continued 

 into the wall of the intestine. 



It has already been mentioned that the intestine is 

 a slender and thin-walled tube, which passes straight 

 through the body almost without change, except that it 

 becomes a little wider and thicker-walled near the vent. 

 Immediately behind the pyloric valves, its surface is quite 

 smooth and soft (figs. 9, 10, and 12, mg), and its floor 

 presents a relatively large aperture, the termination of 

 the bile duct (fig. 12, bd, fig. 10, hp.), on each side. The 

 roof is, as it were, pushed out into a short median pouch 

 or caecum, {cce). Behind this, its character suddenly 

 changes, and six squarish elevations, covered with a 

 chitinous cuticle, encircle the cavity of the intestine (r). 

 From each of these, a longitudinal ridge, corresponding 

 with a fold of the wall of the intestine, takes its rise, and 

 passes, with a slight spiral twist, to its extremity Qig). 

 Each of these ridges is beset with small papillae, and the 

 chitinous lining is continued over the whole to the vent, 

 where it passes into the general cuticle of the integu- 

 ment, just as the lining of the stomach is continuous 

 with the cuticle of the integument at the mouth. The 

 alimentary canal may, therefore, be distinguished into 

 a fore and a hind-gut {hg), which have a thick internal 

 lining of cuticular membrane ; and a very short mid- 

 gut {mg), which has no thick cuticular layer. It will be of 



